1. Field of the Invention
The present invention relates to a catalyst for purifying an exhaust gas emitted from an internal combustion engine of an automobile, etc., and a process for producing the same. Specifically, it relates to an NO.sub.x -storage-and-reduction type catalyst which is optimum for purifying nitrogen oxides (NO.sub.x), included in an exhaust gas emitted from a so-called "lean-burn engine", and a process for producing the same.
2. Description of the Related Art
As a catalyst for purifying an automotive exhaust gas, there has been employed a 3-way catalyst so far which oxidizes CO and hydrocarbons (hereinafter referred to as "HC") and simultaneously reduces NO.sub.x. For example, the 3-way catalyst has been known widely which comprises a heat-resistant substrate formed of cordierite, or the like, a porous catalyst carrier layer formed of .gamma.-alumina, or the like, and formed on the substrate, and a noble metal catalyst ingredient selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), and the like, and loaded on the porous catalyst carrier layer.
From the viewpoint of the global environment protection, carbon dioxide (CO.sub.2), which is emitted from internal combustion engines of automobiles, and so on, is at issue. In order to reduce the carbon dioxide, so-called lean-burn engines are regarded promising. In lean-burn engines, the air-fuel mixture is lean-burned in an oxygen-rich atmosphere (or a fuel-lean atmosphere). The fuel consumption can be reduced because lean-burn engines consume the fuel less. Accordingly, the carbon dioxide, which is emitted from lean-burn engines as one of the burned exhaust gases, can be inhibited from generating.
The conventional 3-way catalysts oxidize CO and HC, and simultaneously reduce NO.sub.x to purify them. The CO, HC and NO.sub.x are produced by burning an air-fuel mixture whose air-fuel ratio is controlled at the theoretical air-fuel ratio (i.e., the stoichiometric point). Consequently, the conventional 3-way catalysts do not have enough activity to remove NO.sub.x, which results from the exhaust gases produced by burning the fuel-lean air-fuel mixture, by reduction in an oxygen-rich atmosphere (or in a fuel-lean atmosphere). Thus, it has been desired to successfully develop an automotive exhaust catalyst and a purifying system which can effectively purify NO.sub.x in an oxygen-rich atmosphere (or in a fuel-lean atmosphere).
Therefore, a system has been developed for lean-burn engines in order to reduce and purify NO.sub.x emitted therefrom. In the system, HC and CO are usually burned in a fuel-lean atmosphere (or an oxygen-rich atmosphere) involving oxygen excessively, and simultaneously NO.sub.x is stored. Accordingly, the exhaust gas is turned into a reducing atmosphere by establishing the stoichiometric point or a fuel-rich atmosphere (or an oxygen-lean atmosphere) temporarily. Thus, NO.sub.x can be reduced and purified. Then, an NO.sub.x -storage-and-reduction tape exhaust gas purifying catalyst has been developed which is appropriate for the system. The NO.sub.x -storage-and-reduction type exhaust gas purifying catalyst employs an NO.sub.x storage member which stores NO.sub.x in a fuel-lean atmosphere (or in an oxygen-rich atmosphere) and releases the stored NO.sub.x at the stoichiometric point or in a fuel-rich atmosphere (or in an oxygen-lean atmosphere).
For example, Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652 proposes a catalyst for purifying an exhaust gas in which an alkaline-earth metal, such as barium (Ba), and Pt are loaded on a porous support, like alumina, etc. Japanese Unexamined Patent Publication (KOKAI) No. 6-31,139 proposes a catalyst for purifying an exhaust gas in which an alkali metal, such as potassium (K), and Pt are loaded on a porous support, like alumina, etc. Japanese Unexamined Patent Publication (KOKAI) No. 5-168,860 proposes a catalyst for purifying an exhaust gas in which a rare earth element, such as lanthanum (La), and Pt are loaded on a porous support, like alumina, etc.
When the aforementioned system employs these catalysts and the air-fuel ratio is controlled from a fuel-lean atmosphere (or an oxygen-rich atmosphere) to the stoichiometric point or a fuel-rich atmosphere (or an oxygen-lean atmosphere) in a pulsating manner, the NO.sub.x is stored in the alkali metal, alkaline-earth metal and rare earth element in an fuel-lean atmosphere (or an oxygen-rich atmosphere), and the stored NO.sub.x is then released at the stoichiometric point or in a fuel-rich atmosphere (or in an oxygen-lean atmosphere). The released NO.sub.x reacts with the reducing components, such as HC and CO, to be purified. Thus, the catalysts can efficiently purify NO.sub.x, included even in the exhaust gases emitted from the lean-burn engines. The alkali metal, alkaline-earth metal and rare earth element, which exhibit the action of storing and releasing NO.sub.x, are collectively referred to as an "NO.sub.x storage member". The application of the NO.sub.x storage member is now under way actively.
In the exhaust gas purifying catalysts, the reaction of purifying NO.sub.x was discovered to comprise the following 3 steps:
a first step of oxidizing NO to NO.sub.x in a fuel-lean atmosphere (or an oxygen-rich atmosphere);
a second step of storing the resulting NO.sub.x on the NO.sub.x storage member; and
a third step of releasing the stored NO.sub.x from the NO.sub.x storage member and reducing the released NO.sub.x on the catalysts at the stoichiometric point or in a fuel-rich atmosphere (or in an oxygen-lean atmosphere).
In order to smoothly propagate the first and second steps, it was further discovered that the noble metal catalyst ingredient, such as Pt, and the NO.sub.x storage member are disposed as near as possible with each other. Hence, in the aforementioned exhaust gas purifying catalysts, the noble metal catalyst ingredient and the NO.sub.x storage member are loaded on the porous support, such alumina, etc., in a coexisting manner.
The fuels, however, include the sulfur components in a trace amount. The sulfur components are oxidized to generate SO.sub.x when the fuels are burned, or are oxidized on the catalyst to generate SO.sub.x. Since SO.sub.x is acidic, there arises the following phenomenon in the NO.sub.x -storage-and-reduction type catalyst: the resulting SO.sub.x reacts with the alkaline NO.sub.x storage member to generate sulfites. As a result, the NO.sub.x storing capability of the NO.sub.x storage member is lost to deteriorate the NO.sub.x purifying performance of the NO.sub.x -storage-and-reduction type catalyst. The disadvantageous phenomenon is called as the "sulfur poisoning of the NO.sub.x storage member".
The NO.sub.x storage member reacts with NO.sub.x to produce nitrates. However, in the presence of SO.sub.x, the NO.sub.x storage member exhibits a property: namely; it is more likely to produce the sulfites than the nitrates. Moreover, when the sulfites are once produced, they are less likely to decompose under ordinary engine operating conditions. Consequently, the NO.sub.x storing capability of the NO.sub.x storage member is less likely to revive. Thus, the NO.sub.x storage member loses the NO.sub.x adsorbing capability gradually by the sulfur poisoning. As a result, the NO.sub.x conversion exhibited by the NO.sub.x -storage-and-reduction type catalyst may be decreased sharply after the NO.sub.x -storage-and-reduction type catalyst is subjected to a durability test.
In order to overcome the drawback, an apparatus has been proposed. In the apparatus, a catalyst capable of storing SO.sub.x is disposed on an upstream side of an exhaust gas passage of a lean-burn engine, and an NO.sub.x -storage-and-reduction type catalyst is disposed on a downstream side of the exhaust gas passage. According to the proposed apparatus, SO.sub.x, included in the exhaust gas, is stored in the upstream-side catalyst in a fuel-lean atmosphere (or an oxygen-rich atmosphere). Thus, the downstream-side catalyst is inhibited from being poisoned by sulfur. Moreover, at the stoichiometric point or in a fuel-rich atmosphere (or an oxygen-lean atmosphere), the stored SO.sub.x is released from the upstream-side catalyst, and the stored NO.sub.x is released from the downstream-side catalyst. Thus, the released SO.sub.x and NO.sub.x are reduced and purified by the HC and CO included in the exhaust gas.
The recent survey, however, has revealed that the reaction of NO.sub.x storage member with SO.sub.x takes place not only in a fuel-lean atmosphere (or an oxygen-rich atmosphere), but also in a fuel-rich atmosphere (or an oxygen-lean atmosphere). Hence, in the proposed apparatus having the aforementioned catalyst arrangement, the stored SO.sub.x is released from the upstream-side catalyst in a fuel-rich atmosphere (or an oxygen-lean atmosphere), and is eventually reacted with the NO.sub.x storage member of the downstream-side catalyst even in a fuel-rich atmosphere (or an oxygen-lean atmosphere). Thus, there arise the problems associated with the sulfur poisoning of the NO.sub.x storage member.
In addition, in the conventional exhaust gas purifying catalysts, Pt is subjected to the granular growth (or sintering) in a fuel-lean atmosphere (or an oxygen-rich atmosphere). Accordingly, there arises a drawback in that the active cites of the catalysts are decreased so that the reactivities are decreased in the above-described first and third steps of the NO.sub.x purifying reaction. As a result, when the conventional catalysts are used at elevated temperatures, the conventional catalysts suffer from the decreased NO.sub.x purifying and durability.
In this instance, Japanese Unexamined Patent Publication (KOKAI) No. 4-122,441 discloses an engineering technique for inhibiting Pt from sintering. According to the publication, the sintering of Pt, which results from the granular growth of alumina, is prohibited by employing alumina which is subjected to a heat treatment in advance. It was discovered, however, that not only the sintering of Pt is resulted from the granular growth of alumina, but also it is further facilitated when the NO.sub.x storage member and Pt are disposed in proximity to each other. In addition, there arises another problem in that the NO.sub.x storage reaction of the second step and the NO.sub.x reduction reaction of the third step are less likely to occur when the NO.sub.x storage member and Pt are separated away from each other by a large distance. If such is the case, the NO.sub.x purifying performance of the NO.sub.x -storage-and-reduction type catalyst is deteriorated eventually.